Various systems are known for displaying visual images of a scene using electromagnetic radiation of a specific spectral region or band. For instance, infrared (IR) devices are employed in numerous applications for both civilian and military purposes. It is also known to observe a scene in an extreme low light environment using light amplification or intensification such as night vision equipment employing image intensifier technology. An example of a night vision device is the night vision goggle designated by the U.S. military as an AN/PVS-7. Another night vision device is described in U.S. Pat. No. 4,463,252. Different devices are needed for displaying scenes in different spectral ranges or bands because different information is conveyed through the different spectra. While various techniques have been employed to combine multi-spectral images of a single scene, they share various disadvantages and deficiencies.
One technique known for combining an infrared image with an image displayed at visible wavelengths is described in U.S. Pat. No. 4,751,571 to Lillquist. The system disclosed in this patent has two separate image paths. One path transmits visible light to an image intensifier while a second path transmits thermal IR to an IR detector. Both the intensified image and the IR image are converted to electronic video signals. The two electronic signals are then mixed at a video mixer and then displayed on a color monitor. The technique described in the Lillquist patent has the disadvantage of requiring that both signals be electronically converted and electronically combined before being displayed to a user. Such electronic conversion will loose the very high resolution of the night vision device. Additionally, the Lillquist approach requires a dedicated system not usable with the vast population of existing night vision devices.
Another image fusion system is described in U.S. Pat. No. 5,035,472 to Hansen. This patent describes a device that conveys the image along two separate paths. One path directs the image to an IR detector which produces an IR image. The IR image is then transmitted to a CRT which displays the image. A second path directs the image of the scene to an image intensifier tube which generates an intensified image which also produces a visual display of the image. The displayed IR and intensified images are optically combined for display to a user. The technique described in the Hansen patent requires both images to be visibly generated before they are optically combined behind the image intensifier. This approach has the disadvantage of requiring a dedicated system not usable with the vast population of existing night vision devices.
Another technique calls for detecting a scene using multiple sensors which convert the images to digital data and then algorithmically combine the data using microelectronic processors. The disadvantages of this approach are that it is time consuming, requires substantial processing power, and the high resolution of a night vision channel cannot be captured electronically.
All of these techniques suffer from the primary disadvantage of not being capable of use with pre-existing direct view devices. For instance, none of these devices is capable of operation with existing night vision equipment.
While these devices and methods have provided a significant improvement over prior approaches, the challenges in the field of electro-optics has continued to increase with demands for more and better techniques having greater flexibility and adaptability. Therefore, a need has arisen for a new modular method for combining multi-spectral images of a scene.